Inductive position indicators which are known in the form either of rotary or linear sensors are used to monitor the relative angular or linear position or movement a first one of two bodies, which are movable with respect to each other, occupies or performs with respect to the second one.
For this purpose a position indicator comprises at least one exciter coil to which an ac voltage is fed in order to generate a magnetic flux. Furtheron, flux guiding means of ferromagnetic material are provided, at least part of which is fixedly connected to one of said two bodies, whereas a measurement coil arrangement which comprises several measurement coils is fixedly connected to the other body. The flux guiding means guide the magnetic flux generated by said exciter coil in such a way that the flux passing through at least one of the measurement coils of said measurement coil arrangement changes if the relative position of said two bodies is changed because of a corresponding movement. The output signals of said measurement coils are used to generate either an ac measurement signal m, the amplitude of which represents the relative position to be monitored or a dc measurement signal M the absolute value of which is a measurement value of said position. In either case the respective analog value can be digitalized in order to obtain the measurement value in the form of a digital word.
One form of circuit arrangement for an inductive position indicator or sensor has a measurement coil arrangement comprising a plurality of measurement coils forming at least first and second groups, each of which supplies a measurement coil signal which changes with the position to be monitored. The circuit arrangement includes a computing or calculating circuit which, to produce a measurement signal, interlinks at least two signals derived from the measurement coil signals, in accordance with a predetermined algorithm. Such a circuit arrangement is used with measurement coil arrangements at which at least two measurement coil signals which can be referred to as a, b can be constantly taken off, and those signals exhibit amplitude responses which are different from each other when the inductive position indicator passes through its measurement range. It is assumed that both multiplicative and also additive interference parameters are incorporated into those two ac voltage signals a, b in corresponding fashion, and the influence thereof on the measurement result is to be reduced by using as the measurement signal a quotient m=(a-b)/(a+b) which is formed by means of the computing circuit in a procedure which will be described in greater detail hereinafter.
Complete elimination of the additive interference parameters from the quotient m is possible only when, as described in European patent application No. 92 112550.6, the measurement coil arrangement supplies signals from which additive interference phenomena have been eliminated by a difference-forming operation. If that is the case, one of the following quotients: EQU m=.DELTA.a/(.DELTA.a+.DELTA.b) (1)
or EQU m'=(.DELTA.a-.DELTA.b)/(.DELTA.a+.DELTA.b) (2)
is advantageously used as the measurement signal, from which multiplicative interference phenomena are also totally eliminated by virtue of the quotient-forming operation.
A computing circuit for producing the quotients m and m' respectively can be so designed that disposed on the downstream side of first and second inputs to which the respective signals .DELTA.a and .DELTA.b are constantly supplied is an input amplifier whose output is connected to one end or the other of a chain of resistors comprising resistors of exactly the same size. Each of the two end points of the chain of resistors and each connecting point between each two successive resistors can be connected to a common output terminal by way of its own controllable switch. Of those controllable switches, there is only ever one that is closed, while all the others are open. Which of the switches is closed and which are open is established by a digital word which is produced by a counter and which serves to actuate the controllable switches.
The counter counts the oscillations of a voltage-controlled oscillator whose control input is connected to the output terminal of the chain of resistors. It is only when a voltage value of zero appears at that output that the oscillator stops and the count value attained by the counter represents the required measurement value m or m' respectively.
In other words: by means of the above-described computing circuit which is in the form of a regulating loop, the input signals .DELTA.a and .DELTA.b are weighted with the factors m and 1-m, and 1-m' and 1+m' respectively, and then summed. The value m and m' respectively is varied until the sum signal is equal to 0. That corresponds to solving the above equation (1) in accordance with: EQU .DELTA.a' (1-m)-m.cndot..DELTA.b=0 (3),
in which respect a particular advantage is to be seen in the fact that m is also available as a digital word.
In order to achieve a high degree of resolution, instead of a resistor series circuit, it is possible to provide two resistor chains which are arranged in hierarchical graded relationship, of which the first receives the pre-amplified input signals .DELTA.a, .DELTA.b in the above-described manner. The switches which are associated with the tapping points of that first resistor chain lead alternately to one end point and the other of the second resistor chain which is also connected by switches to the output terminal of this arrangement, in the above-described manner. Of the switches associated with the first resistor chain, there are always two immediately successive switches that are closed simultaneously, while all others are open. Cyclic actuation of the pairs of switches in the closed condition by means of the most significant bits of the digital word supplied by the counter provides a coarse division effect. The switches disposed on the output side of the second resistor chain are individually closed in succession for each closed pair of switches of the first chain, by means of the least significant bits of the digital word, as was described above in relation to the switches of the individual resistor chains. The second resistor chain therefore represents a fine division operation. With such a graded arrangement, for example with 2.times.64 resistors, it is possible to achieve a level of resolution of 12 bits, for which 1024 resistors would be necessary when using a single resistor series circuit.
It will be appreciated however that this principle of hierarchical grading of a plurality of resistor chains cannot be prolonged just as desired, as a specific operational amplifier is required for decoupling of each one of the lines which jointly go to the subsequent resistor chains, so that the level of expenditure required in that respect quickly exceeds the savings achieved in terms of resistors.